In my research on large ductile iron castings for marine applications, I have encountered several typical defects that significantly impact the quality and performance of these components. Defects such as shrinkage porosity, graphite flotation, and broken graphite are prevalent in thick-section ductile iron castings, often leading to high rejection rates. This study aims to investigate the influencing factors, particularly carbon equivalent (CE) and trace elements, on these defects through systematic experimentation and analysis. The findings are intended to optimize the melting and casting processes for producing high-integrity ductile iron castings.
Large marine diesel engine blocks, typically made of ductile iron casting with material grade QT400-18AR, have complex geometries with wall thicknesses ranging from 10 mm to 180 mm and weights up to 40 tons. These ductile iron castings require stringent mechanical properties, including high tensile strength, elongation, and impact resistance, along with rigorous non-destructive testing. However, the inherent characteristics of ductile iron casting, such as mushy solidification and graphite expansion, often lead to defects that compromise quality. My focus is on understanding how CE and additives like antimony (Sb) can mitigate these issues in ductile iron casting production.
To conduct this study, I designed a step-block specimen that simulates the varying wall thicknesses of typical ductile iron casting components. The step-block had sections with thicknesses of 40 mm, 80 mm, 120 mm, 180 mm, and 220 mm, each 250 mm in length (except the last section at 300 mm). This design allows for evaluating defect formation across different cooling rates and solidification times, which are critical in ductile iron casting. The gating system was planned to ensure consistent filling, and the specimens were poured using iron melts with controlled compositions.
The raw materials for the ductile iron casting melts consisted of 65% high-purity pig iron and 35% high-quality steel scrap, with carburizer added to adjust the carbon content. Melting was carried out in a 1.5-ton medium-frequency induction furnace, with a maximum temperature of 1500°C. The treatment process involved a sandwich method for spheroidization using rare earth magnesium alloy and inoculation with silicon barium alloy, both common in ductile iron casting production. Each batch treated 1 ton of molten iron, and four molds were poured per batch at different pouring temperatures: 1370°C, 1350°C, 1330°C, and 1310°C. Thermal analysis was performed to record cooling curves, which provide insights into the solidification behavior of ductile iron casting.
After casting, the step-blocks were shaken out, cleaned by shot blasting, and sectioned for analysis. Macroscopic examination was conducted to assess shrinkage porosity, while metallographic samples were taken from the upper and central regions of each section to observe graphite morphology and matrix structure. The chemical compositions of the melts after treatment are summarized in Table 1, showing variations in CE and trace elements like Sb.
| Scheme | CE (%) | Mn (%) | P (%) | S (%) | Sb (%) |
|---|---|---|---|---|---|
| 1 | 4.01 | 0.22 | 0.015 | 0.010 | 0.000 |
| 2 | 4.21 | 0.22 | 0.015 | 0.010 | 0.000 |
| 3 | 4.42 | 0.23 | 0.015 | 0.012 | 0.000 |
| 4 | 4.61 | 0.22 | 0.015 | 0.012 | 0.000 |
| 5 | 4.42 | 0.23 | 0.016 | 0.012 | 0.005 |
| 6 | 4.42 | 0.23 | 0.015 | 0.012 | 0.005 |
| 7 | 4.32 | 0.23 | 0.014 | 0.013 | 0.007 |
| 8 | 4.32 | 0.23 | 0.015 | 0.013 | 0.007 |
The carbon equivalent (CE) is a critical parameter in ductile iron casting, defined as: $$CE = C + \frac{1}{3}(Si + P)$$ where C, Si, and P are the weight percentages of carbon, silicon, and phosphorus, respectively. This formula helps predict the graphite formation and shrinkage tendencies in ductile iron casting. In my experiments, I varied CE from 4.01% to 4.61% to study its effects on defects.
From the thermal analysis curves, I derived parameters such as G1 and G3, which represent the austenite precipitation stage and the pure graphite precipitation stage, respectively. These parameters are indicative of the solidification characteristics in ductile iron casting. For instance, a higher G1 value suggests a larger austenite formation range, associated with hypoeutectic compositions and increased shrinkage tendency. Conversely, a higher G3 value indicates strong graphiteification ability, which can counteract shrinkage through expansion. The relationship can be expressed as: $$G1 \propto \frac{1}{CE} \quad \text{and} \quad G3 \propto CE$$ This implies that as CE increases, G1 decreases and G3 increases, reducing the risk of shrinkage in ductile iron casting.
The results from macroscopic examination are summarized in Table 2, showing the correlation between CE, wall thickness, and shrinkage porosity in ductile iron casting specimens poured at 1370°C.
| CE (%) | 40 mm | 80 mm | 120 mm | 180 mm | 220 mm |
|---|---|---|---|---|---|
| 4.01 | None | None | None | Minor | Significant |
| 4.21 | None | None | None | Minor | Significant |
| 4.42 | None | None | None | None | None |
| 4.61 | None | None | None | None | None |
As observed, for a given CE, shrinkage porosity tends to increase with wall thickness due to prolonged solidification times and inadequate liquid feeding in ductile iron casting. However, at higher CE levels, such as 4.42% and above, shrinkage is effectively eliminated even in thick sections, highlighting the self-feeding capability of ductile iron casting through graphite expansion. This is a key advantage in producing sound ductile iron castings.
Graphite flotation, another common defect in ductile iron casting, occurs when graphite nodules float to the upper surfaces of thick sections due to their lower density. Table 3 presents the incidence of graphite flotation relative to CE and wall thickness.
| CE (%) | 40 mm | 80 mm | 120 mm | 180 mm | 220 mm |
|---|---|---|---|---|---|
| 4.01 | None | None | None | Minor | Minor |
| 4.21 | None | None | None | Minor | Minor |
| 4.42 | None | None | None | Moderate | Moderate |
| 4.61 | None | Minor | Moderate | High | High |
Clearly, higher CE and greater wall thickness promote graphite flotation in ductile iron casting. At CE = 4.61%, significant flotation occurs in sections above 120 mm, compromising the mechanical properties of ductile iron casting. Therefore, balancing CE to avoid both shrinkage and flotation is crucial. Based on my data, for pouring temperatures around 1370°C and wall thicknesses up to 120 mm, the optimal CE range for ductile iron casting is 4.01% to 4.42%.
Broken graphite, also known as chunky graphite, is a detrimental defect in thick-section ductile iron casting, characterized by irregular graphite clusters that weaken the material. Table 4 shows the occurrence of broken graphite with varying CE and wall thickness.
| CE (%) | 40 mm | 80 mm | 120 mm | 180 mm | 220 mm |
|---|---|---|---|---|---|
| 4.01 | None | None | None | Minor | Minor |
| 4.21 | None | None | None | Minor | Minor |
| 4.42 | None | None | None | Moderate | Moderate |
| 4.61 | None | None | Minor | High | High |
Broken graphite becomes pronounced at CE above 4.42% in thick sections, indicating that excessive graphiteification can lead to unstable growth in ductile iron casting. To address this, I investigated the role of trace elements, particularly antimony (Sb), in suppressing broken graphite. Sb is known to dissolve in austenite, hindering carbon diffusion and slowing graphite growth, thereby stabilizing the nodular structure in ductile iron casting.
In additional experiments, I added Sb at levels of 0.005% and 0.007% to melts with CE of 4.42% and 4.32%, respectively. Some specimens were also cast with chills (cold iron plates) placed at the bottoms of 80 mm and 120 mm sections to enhance cooling. The results, summarized in Table 5, demonstrate the combined effects of Sb and chills on graphite morphology and mechanical properties in ductile iron casting.
| Scheme | CE (%) | Sb (%) | Chill | Graphite Nodularity (%) | Graphite Size (Grade) | Tensile Strength (MPa) | Elongation (%) | Matrix Structure |
|---|---|---|---|---|---|---|---|---|
| 5 | 4.42 | 0.005 | No | 80 | 5 | 426 | 18 | F + 15% P |
| 6 | 4.42 | 0.005 | Yes | 90 | 6 | 415 | 20 | F + <5% P |
| 7 | 4.32 | 0.007 | No | 80 | 5 | 457 | 12 | F + 35% P |
| 8 | 4.32 | 0.007 | Yes | 80 | 5 | 464 | 11 | F + 45% P |
The data indicates that adding Sb effectively prevents broken graphite in ductile iron casting, with no occurrences observed in any sections. Moreover, the use of chills significantly improves graphite nodularity and refines graphite size from grade 5 to grade 6 in ductile iron casting. However, higher Sb content (0.007%) increases pearlite fraction in the matrix, which is undesirable for ferritic ductile iron casting requiring high ductility. This can be described by the equation: $$\% \text{Pearlite} \propto [Sb]$$ where [Sb] is the antimony concentration. Thus, careful control of Sb addition is essential in ductile iron casting production.
The mechanism behind Sb’s effect can be modeled using diffusion theory. The growth rate of graphite nodules in ductile iron casting is governed by carbon diffusion, and Sb atoms act as barriers. The diffusion coefficient D in the presence of Sb can be approximated as: $$D = D_0 \exp\left(-\frac{Q}{RT}\right) \cdot \frac{1}{1 + k[Sb]}$$ where \(D_0\) is the pre-exponential factor, \(Q\) is activation energy, \(R\) is gas constant, \(T\) is temperature, and \(k\) is a constant. This reduction in D slows graphite growth, preventing breakdown in ductile iron casting.
Chills enhance cooling rates, which modify the solidification parameters in ductile iron casting. The cooling rate \( \dot{T} \) can be related to the chill thickness \( t_c \) and wall thickness \( t_w \) by: $$\dot{T} = \frac{K}{(t_c + t_w)^n}$$ where \(K\) and \(n\) are material constants. Faster cooling promotes finer graphite and reduces segregation, benefiting ductile iron casting quality.
Based on these findings, I optimized the melting process for large ductile iron castings. The revised parameters include: 65% pig iron and 35% steel scrap charge; target CE of 4.2% to 4.5%; manganese below 0.25%; phosphorus below 0.05%; sulfur below 0.02%; and controlled Sb addition. Spheroidization was performed with 1.2% low-rare earth magnesium alloy, and inoculation with 0.6% to 0.7% silicon barium alloy at treatment temperatures of 1470°C to 1490°C. This optimized approach aims to produce defect-free ductile iron castings.
To validate the optimization, I applied this process to produce five engine block ductile iron castings. The mechanical properties and metallographic results from attached test blocks are shown in Table 6, confirming that all specifications were met for ductile iron casting components.
| Sample | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Impact Energy (J) | Hardness (HB) | Graphite Nodularity (Grade) | Graphite Size (Grade) | Ferrite Fraction (%) |
|---|---|---|---|---|---|---|---|---|
| P01 | 407 | 274 | 25.5 | 20 | 143 | 2 | 6 | >95 |
| P02 | 402 | 265 | 26.5 | 18 | 141 | 2 | 6 | >95 |
| P03 | 406 | 272 | 27.5 | 19 | 143 | 2 | 6 | >95 |
| E01 | 401 | 279 | 27.5 | 21 | 141 | 2 | 6 | >95 |
| E01 Body | 402 | 263 | 27.0 | 18 | 148 | 2-3 | 5 | >95 |
| E02 | 407 | 306 | 30.0 | 16 | 180 | 2 | 6 | >95 |
These results demonstrate that the optimized process consistently produces high-quality ductile iron castings with excellent mechanical properties and minimal defects. The ductile iron casting components passed all non-destructive tests, including radiography, ultrasonics, and penetrant testing, as well as hydrostatic pressure tests.
In summary, my research on ductile iron casting has shown that carbon equivalent plays a pivotal role in defect formation. For large ductile iron castings, a CE range of 4.2% to 4.5% is optimal to balance shrinkage porosity and graphite flotation. Trace elements like antimony are effective in preventing broken graphite in ductile iron casting, but their concentration must be controlled to avoid excessive pearlite formation. Additionally, the use of chills improves graphite morphology and refining in ductile iron casting. These insights contribute to the advancement of ductile iron casting technology, enabling the production of reliable and high-performance components for marine applications. Future work could explore other trace elements and cooling techniques to further enhance ductile iron casting quality.
The study underscores the importance of integrated process control in ductile iron casting, from melt composition to solidification management. By applying these principles, manufacturers can reduce defects and improve the consistency of ductile iron casting products. As the demand for large and complex ductile iron castings grows, such research will continue to drive innovations in the field, ensuring that ductile iron casting remains a preferred material for critical engineering applications.